Loss of Estrogen-Related Receptor Alpha Facilitates Angiogenesis in Endothelial Cells.

Estrogen-related receptors (ERRs) have emerged as major metabolic regulators in various tissues. However, their expression and function in the vasculature remains unknown. Here, we report the transcriptional program and cellular function of ERRα in endothelial cells (ECs), a cell type with a multifaceted role in vasculature. Of the three ERR subtypes, ECs exclusively express ERRα. Gene expression profiling of ECs lacking ERRα revealed that ERRα predominantly acts as a transcriptional repressor, targeting genes linked with angiogenesis, cell migration, and cell adhesion. ERRα-deficient ECs exhibit decreased proliferation but increased migration and tube formation. ERRα depletion increased basal as well as vascular endothelial growth factor A (VEGFA)- and ANG1/2-stimulated angiogenic sprouting in endothelial spheroids. Moreover, retinal angiogenesis is enhanced in ERRα knockout mice compared to that in wild-type mice. Surprisingly, ERRα is dispensable for the regulation of its classic targets, such as metabolism, mitochondrial biogenesis, and cellular respiration in the ECs. ERRα is enriched at the promoters of angiogenic, migratory, and cell adhesion genes. Further, VEGFA increased ERRα recruitment to angiogenesis-associated genes and simultaneously decreased their expression. Despite increasing its gene occupancy, proangiogenic stimuli decrease ERRα expression in ECs. Our work shows that endothelial ERRα plays a repressive role in angiogenesis and potentially fine-tunes growth factor-mediated angiogenesis.

Genomic insights of protein arginine methyltransferase Hmt1 binding reveals novel regulatory functions.

BACKGROUND: Protein arginine methylation is a post-translational modification involved in important biological processes such as transcription and RNA processing. This modification is catalyzed by both type I and II protein arginine methyltransferases (PRMTs). One of the most conserved type I PRMTs is PRMT1, the homolog of which is Hmt1 in Saccharomyces cerevisiae. Hmt1 has been shown to play a role in various gene expression steps, such as promoting the dynamics of messenger ribonucleoprotein particle (mRNP) biogenesis, pre-mRNA splicing, and silencing of chromatin. To determine the full extent of Hmt1's involvement during gene expression, we carried out a genome-wide location analysis for Hmt1. RESULTS: A comprehensive genome-wide binding profile for Hmt1 was obtained by ChIP-chip using NimbleGen high-resolution tiling microarrays. Of the approximately 1000 Hmt1-binding sites found, the majority fall within or proximal to an ORF. Different occupancy patterns of Hmt1 across genes with different transcriptional rates were found. Interestingly, Hmt1 occupancy is found at a number of other genomic features such as tRNA and snoRNA genes, thereby implicating a regulatory role in the biogenesis of these non-coding RNAs. RNA hybridization analysis shows that Hmt1 loss-of-function mutants display higher steady-state tRNA abundance relative to the wild-type. Co-immunoprecipitation studies demonstrate that Hmt1 interacts with the TFIIIB component Bdp1, suggesting a mechanism for Hmt1 in modulating RNA Pol III transcription to regulate tRNA production. CONCLUSIONS: The genome-wide binding profile of Hmt1 reveals multiple potential new roles for Hmt1 in the control of eukaryotic gene expression, especially in the realm of non-coding RNAs. The data obtained here will provide an important blueprint for future mechanistic studies on the described occupancy relationship for genomic features bound by Hmt1.

Recruitment of Rpd3 to the telomere depends on the protein arginine methyltransferase Hmt1.

In the yeast Saccharomyces cerevisiae, the establishment and maintenance of silent chromatin at the telomere requires a delicate balance between opposing activities of histone modifying enzymes. Previously, we demonstrated that the protein arginine methyltransferase Hmt1 plays a role in the formation of yeast silent chromatin. To better understand the nature of the Hmt1 interactions that contribute to this phenomenon, we carried out a systematic reverse genetic screen using a null allele of HMT1 and the synthetic genetic array (SGA) methodology. This screen revealed interactions between HMT1 and genes encoding components of the histone deacetylase complex Rpd3L (large). A double mutant carrying both RPD3 and HMT1 deletions display increased telomeric silencing and Sir2 occupancy at the telomeric boundary regions, when comparing to a single mutant carrying Hmt1-deletion only. However, the dual rpd3/hmt1-null mutant behaves like the rpd3-null single mutant with respect to silencing behavior, indicating that RPD3 is epistatic to HMT1. Mutants lacking either Hmt1 or its catalytic activity display an increase in the recruitment of histone deacetylase Rpd3 to the telomeric boundary regions. Moreover, in such loss-of-function mutants the levels of acetylated H4K5, which is a substrate of Rpd3, are altered at the telomeric boundary regions. In contrast, the level of acetylated H4K16, a target of the histone deacetylase Sir2, was increased in these regions. Interestingly, mutants lacking either Rpd3 or Sir2 display various levels of reduction in dimethylated H4R3 at these telomeric boundary regions. Together, these data provide insight into the mechanism whereby Hmt1 promotes the proper establishment and maintenance of silent chromatin at the telomeres.

Proteomic analysis of interactors for yeast protein arginine methyltransferase Hmt1 reveals novel substrate and insights into additional biological roles.

Protein arginine methylation is a PTM catalyzed by an evolutionarily conserved family of enzymes called protein arginine methyltransferases (PRMTs), with PRMT1 being the most conserved member of this enzyme family. This modification has emerged to be an important regulator of protein functions. To better understand the role of PRMTs in cellular pathways and functions, we have carried out a proteomic profiling experiment to comprehensively identify the physical interactors of Hmt1, the budding yeast homolog for human PRMT1. Using a dual-enzymatic digestion linear trap quadrupole/Orbitrap proteomic strategy, we identified a total of 108 proteins that specifically copurify with Hmt1 by tandem affinity purification. A reverse coimmunoprecipitation experiment was used to confirm Hmt1's physical association with Bre5, Mtr4, Snf2, Sum1, and Ssd1, five proteins that were identified as Hmt1-specific interactors in multiple biological replicates. To determine whether the identified Hmt1-interactors had the potential to act as an Hmt1 substrate, we used published bioinformatics algorithms that predict the presence and location of potential methylarginines for each identified interactor. One of the top hits from this analysis, Snf2, was experimentally confirmed as a robust substrate of Hmt1 in vitro. Overall, our data provide a feasible proteomic approach that aid in the better understanding of PRMT1's roles within a cell.

Protein arginine methylation facilitates cotranscriptional recruitment of pre-mRNA splicing factors.

Cotranscriptional recruitment of pre-mRNA splicing factors to their genomic targets facilitates efficient and ordered assembly of a mature messenger ribonucleoprotein particle (mRNP). However, how the cotranscriptional recruitment of splicing factors is regulated remains largely unknown. Here, we demonstrate that protein arginine methylation plays a novel role in regulating this process in Saccharomyces cerevisiae. Our data show that Hmt1, the major type I arginine methyltransferase, methylates Snp1, a U1 small nuclear RNP (snRNP)-specific protein, and that the mammalian Snp1 homolog, U1-70K, is likewise arginine methylated. Genome-wide localization analysis reveals that the deletion of the HMT1 gene deregulates the recruitment of U1 snRNP and its associated components to intron-containing genes (ICGs). In the same context, splicing factors acting downstream of U1 snRNP addition bind to a reduced number of ICGs. Quantitative measurement of the abundance of spliced target transcripts shows that these changes in recruitment result in an increase in the splicing efficiency of developmentally regulated mRNAs. We also show that in the absence of either Hmt1 or of its catalytic activity, an association between Snp1 and the SR-like protein Npl3 is substantially increased. Together, these data support a model whereby arginine methylation modulates dynamic associations between SR-like protein and pre-mRNA splicing factor to promote target specificity in splicing.